The invention relates to an improved eyepiece for astronomic positioning and, more particularly, to such an eyepiece having electronic switching means for illuminating the reticle of a survey instrument at timed intervals to observe the position of a star with respect to the illuminated reticle.
It is known in the art relating to astronomic positioning to estimate the time of transit of a star across particular lines on the reticle of a viewing instrument. The Hunter shutter eyepiece described in Clark, "Plane and Geodetic Surveying ", Vol. II, 5th Edition, pgs. 62-63, is one device that has been employed to reduce the personal measurement errors that typically characterize the observations of individual observers.
The Hunter shutter eyepiece may be attached to a viewing instrument such as a geodetic theodolite to automatically time successive rapid observations of the movement of a star and to thereby reduce the timing inaccuracies that result when an individual observer is required to independently observe both the position and corresponding transition time of the star.
The automatic timing of observations is performed by a movable shutter that is positioned in front of a glass plate having an engraved reticle and positioned in the focal plane of the objective of the viewing instrument. In operation, every three seconds the shutter is opened for 70 to 100 milliseconds to expose the star and to thereby allow the observer sufficient time to observe the position of the star relative to the reticle. Thereafter, the observer notes the time of the opening of the shutter and the corresponding estimated position of the star.
From the recorded data, a mean scale reading over a mean observation time may be computed for a corresponding series of rapid observations of the star. Thereafter, the time of the passage of the star over a known altitude or a vertical plane may be determined.
Since the shutter of the Hunter eyepiece is a mechanical device, the relatively rapid operation of the shutter results in vibrations that are transmitted to the associated viewing instrument. Therefore, the Hunter eyepiece is not suitable for operation with viewing instruments such as astrolabes that employ mercury pools or pendulum systems that may be sensitive to such vibrations.
In addition, since the apparatus of the Hunter eyepiece necessarily employs rapidly moving parts, it is prone to the various types of mechanical failure that are associated with such mechanisms.
Also, since the shutter of the eyepiece operates to pass light from the target star for a very short period of time, the amount of light that is received by the viewing eye is correspondingly small and therefore, it is correspondingly difficult for the eye to perceive a star of relatively low intensity or a star having a relatively high background lighting.
Accordingly, it is an object of the invention to provide an improved eyepiece for astronomic positioning having the advantages of the Hunter eyepiece but having no undesirable operational vibrations.
A further object of the invention is to provide such an improved eyepiece that will operate to continuously pass light from the target star to enable the observer to perceive stars of low intensity or stars having a high background lighting.
Another object of the invention is to provide such an eyepiece that operates electronically with no moving parts to provide increased reliability.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.